Particulate emissions from a production gasoline direct injection spark ignition engine were studied under a typical cold-fast-idle condition (1200 rpm, 2 bar NIMEP). The particle number (PN) density in the 22 to 365 nm range was measured as a function of the injection timing with single pulse injection and with split injection. Very low PN emissions were observed when injection took place in the mid intake stroke because of the fast fuel evaporation and mixing processes which were facilitated by the high turbulent kinetic energy created by the intake charge motion. Under these conditions, substantial liquid fuel film formation on the combustion chamber surfaces was avoided. PN emissions increased when injection took place in the compression stroke, and increased substantially when the fuel spray hit the piston. A conceptual model was established for the particulate matter (PM) formation process in which PM is formed by pyrolysis after the normal premixed flame passage in fuel rich plumes originating from liquid films on the cylinder walls. The pyrolysis process is supported by heat conducted from the hot burned gases outside the plume and by the energy released by the pyrolysis reactions. Thus, the “pool fire” often observed is not a diffusion flame since the small amount of residual oxygen in the burned gases cannot support such a flame. The luminosity is radiation from the hot soot particles which are not oxidized after being formed in the pyrolysis reactions. This model was supported by the PN data obtained from sweeping the charge equivalence ratio from lean to rich.